Summer and winter variations of BTEX concentrations in an oil refinery complex and health risk assessment based on Monte-Carlo simulations

The summer and winter concentrations of BTEX pollutants were investigated in various workplaces of an oil Refinery, Iran. In total 252 air samples from the breathing zones of the following employees were collected: supervisors, safetymen, repairmen, site men, and all workers. Carcinogenic and non-carcinogenic risk values were calculated based on the USEPA methodology using Monte Carlo simulations. BTEX concentrations were higher in the summer than in the winter season for all workstations, especially for toluene and ethylbenzene. The mean values of exposure to benzene for repairmen and site men were higher than threshold limit value of 1.60 mg/m3 for both seasons. Non-carcinogenic risk (HQ) values calculated for summer season for benzene, ethylbenzene, and xylene in all workstations, as well as for toluene for repairmen and site men exceeded acceptable level of 1. In winter season the mean HQ values for benzene and xylene in all workstations, for toluene for repairmen and site men, and for ethylbenzene for supervisors, repairmen, and site men were also > 1. For all workstations definite carcinogenic risk was indicated as calculated LCR values for benzene and ethylbenzene exposure were higher than 1 × 10–4 in both summer and winter seasons.

Occupational exposure characteristic. The cross-sectional study was performed on the employees of the distillation units of an Oil Refinery, Iran. The inclusion criteria for the refinery's workers to participate in the study was to have at least 12 months of work experience and to be exposed to BTEX compounds via inhalational pathway based on preliminary examination. The number of subjects required in this study was computed using Cochran equation with error level of 0.05 and was equal to 84. From each workstation three air samples were taken for the each participant in order to compute the time weighted average (TWA) during the shiftwork exposure, thus in total 252 samples were collected. To check the impact of the used methodology and equipment on pollutant concentration measured, also 25 blank samples were taken in the same conditions as the main samples. Prior the investigations participants signed the informed consent. Questionnaire surveys performed included answering the questions on age, weight, work experience, number of working days per year, and exposure duration per day.
Air sampling and exposure assessment. Sampling method. Concentrations of BTEX compounds, were measured using the National Institute for Occupational Safety and Health (NIOSH) 1501 method 28 . The measurements were conducted during winter and summer of the year 2021 29 . To collect the air samples in the working environments, the adsorbent tube containing activated coconut charcoal (front: 100 mg and rear: 50 mg) produced by SKC Inc. were used 30 . The adsorbent tubes were placed on the workers' collars within the breathing zone, and a calibrated SKC pump with a flow rate from 50 to 200 ml/min was applied to pass air through www.nature.com/scientificreports/ the tube 30 . Sampling was carried out by a personal sampling pump, model AirChek TOUCH (5-5000 mL/min, SKC, Inc.). Initially, a pretest was performed to reveal the breakthrough volume. For each subject three indoor air samples were gathered to cover the work shift, thus measurements were performed at the beginning, in the middle, and at the end of the work shift of the participants included 31 . The sampling time of each sample was between 80 and 120 min. After sampling, the plastic caps were used to seal adsorbent tubes. The samples were transferred to a laboratory in a cool box to prevent the potential leakage. The atmospheric conditions of the sampling locations were measured using WBGT device (Tenmars electronics CO., Taiwan) and the results of air temperature and relative humidity measurements were recorded.
Sample preparation and analysis. In this step, the adsorbent tubes were broken and the pollutants were chemically desorbed by 1 ml of carbon disulfide solvent in extraction vials. After 60 min, 1 μl of this solution was injected by a syringe into a gas chromatograph with a capillary column equipped with a GC-FID flame ionization detector (GC 7890 Agilent) 30 . Helium (flow rate of 1 ml/min) was exploited as carried gas. Moreover, 25 blank samples were analyzed to eliminate errors during sampling and analyzing steps.
Quality assurance/quality control (QA/QC). Both the air and blank samples were placed in a cold box with ice packs (~ 4 °C) after sampling and during the transfer to the laboratory. After reaching the laboratory, samples were stored in the temperature of 4 °C before the analyses. The sampling tubes were broken at the sampling site, then their ends were sealed using plastic caps, and transmission and analyses were performed using the similar methods in the case of both main and blank samples. Extraction solvent of carbon disulfide (CS 2 ) was injected and analyzed using the gas chromatography flame ionization detector (GC-FID) three times to determine BTEX contents. The limit of detection (LOD) was determined as follows: LOD = 3.3 × standard deviation (SD) of the blanks (slope of the calibration curve) 32  where C is the concentration of pollutant (mg/m 3 ), ET is exposure time (hours/day), ED is exposure duration (years), EF is exposure frequency (days/year), AT is averaging time (ED in years × 365 days/year). The values of exposure and toxicological parameters used in the study are presented in Table 2.
If HQ values are < 1 it indicates the lack of adverse non-carcinogenic health effects, while HQ values ≥ 1 point the presence of non-carcinogenic health effects 35 .
Carcinogenic risk assessment. For determination of the carcinogenic risk of pollutants also the USEPA method was used. In this method the carcinogenic risk is described by the lifetime cancer risk (LCR) index. As no Inhalational Unit Risk (IUR) values were available for toluene, ethylbenzene, and xylene, instead of exposure concentration (EC), chronic daily intake (CDI) was used in order to receive the unified results. The LCR values were calculated according to the Eq. (3): where CDI is chronic daily intake (mg/kg-day) and SF is cancer slope factor ((mg/kg-day) -1 ) . Values of SF was taken from the IRIS toxicological database 36,37 , however only SF values for benzene and ethylbenzene were available. The CDI values were computed using the Eq. (4) 34 .
where C is concentration of pollutant (mg/m 3 ), IR is inhalation rate (m 3 /day), ED is exposure duration (years), EF is exposure frequency (days/year), BW is body weight (kg), AT is averaging time (days). The values of exposure and toxicological parameters used in the study are presented in Table 1.
Monte-Carlo simulations. To achieve as reliable and realistic results as possible regarding the uncertainty and variability of the measurements performed, mathematical modeling is recommended 38 . Monte Carlo simulation (MCS), as a probabilistic and statistical-mathematical approach, that uses a combination of simulations, in order to determine uncertainty by the Monte Carlo method. The computations were conducted by 1000 iterations, and the results were estimated with a confidence degree between 1 and 99% 39 . In this study, Crystal Ball software (version 11.1.2.4, Oracle, Inc., USA) was exploited for Monte Carlo simulations (MCS).

Statistical analysis.
In order to check the normality of the data distribution, the Kolmogorov-Smirnov test was used. Given the lack of normal distribution of the variables, Friedman test was used to examine difference of mean values of exposure to BTEX in two investigated seasons, namely summer and winter. Other values of descriptive statistics were reported using SPSS software.

Ethics statement. The Research Ethics Committee of Shahrekord University of Medical Sciences (SKUMS)
granted ethics approval for this study (IR.SKUMS.REC.1401.021). Also, we confirm that all methods were performed in accordance with the relevant guidelines and regulations. Consent to participate. All of the subjects had full consent to participate in the current study.

Results and discussion
Atmospheric conditions of the performed measurements. In our studies measurements of air temperature, relative humidity, and wind speed were performed along with the BTEX concentrations. In the summer season the mean value of air temperature was equal to 40.4 °C (standard deviation, SD 1.89 °C). In the winter season the mean air temperature was reported to be 15.3 °C (SD 1.64 °C). The mean value of the relative humidity in the summer season was equal to 45.1% with SD of 5.11%. In the winter season the mean value of the relative humidity was equal to 20.5% (SD 2.12%). The measurements of wind speed in summer season indicated the mean value equal to 1.88 m/s (SD 1.04 m/s). In the winter season the mean value of wind speed was reported to be 1.96 m/s, with the SD value of 0.89 m/s. To sum up, the measurements revealed that temperature and relative humidity at workplaces were much higher in summer than in winter seasons, while wind speed was higher in winter than in summer season.

Seasonal variations of BTEX concentrations. The concentrations of BTEX in the breathing zones of
the employees in the various workstations in summer and winter seasons are presented in Fig. 1. It can be observed that analyzed BTEX concentrations were higher in the summer than in the winter season for all workstations and were particularly visible for toluene and ethylbenzene. It can be related with the fact that the mean temperature in the investigated production process was 40.4 °C in summer and 25.3 °C in winter affecting the higher vaporization of the BTEX compounds. Also, the relative humidity in summer season, which mean value was equal to 45.1%, while in winter season it was equal to 20.5% also might affected the increased BTEX emissions. The results of Friedman test revealed that exposure to xylene in supervisors and exposure to BTEX in safetymen, repairmen, and site men were significantly higher in summer season compered to winter seasons (P < 0.045) ( Table 2). Moreover, it was revealed that the exposure in particular workstations was ordered decreasingly as follows: repairmen > site men > safety men > supervisors. In the particular groups of employees, the mean BTEX values in summer and winter seasons in the breathing zones were as follows ( 40 performed in urban areas of Malaysia indicated positive correlations of BTEX concentrations with relative humidity, however negative correlations with wind speed, solar radiation, and air temperature. Study of Popitanu et al. 41 in the ambient air of Arad city, Romania, revealed that the increase of the BTEX concentrations in winter season was related with the usage of fuels in the central heating season. Also, due to the higher reaction rates with OH radicals in the atmosphere BTEX were removed faster in summer than in winter 41 . The studies of Bodor et al. 4 on seasonal differences in air pollution around Ploiesti oil refining complex in Romania also pointed that the highest.   42 in the forest site in the Western Mediterranean Basin revealed that almost all analyzed VOCs had higher average mixing ratios during the summer than the winter season, except for VOCs that are linked to anthropogenic sources: for them lower (benzene) or similar (toluene) mixing ratios in summer than in winter were stated.
For all the workstations the decreasing order of BTEX was as follows: toluene > ethylbenzene > xylene > benzene. These finding stay in line with investigations of Rajabi et al. 43 indicating that among all VOCs emitted from crude oil, toluene, benzene, hexane, heptane, cyclohexane, and pentane were found to be highly detected and concentrated compounds. Investigations of Ercan et al. 44 in Istanbul, Turkey revealed that BTEX concentrations in industrial zones were higher than those measured in suburban locations. Investigations of Bretón et al. 45 on BTEX concentrations in an urban site located in the vicinity of an oil storage-distribution facility in Paraiso, Tabasco, Mexico revealed that lower BTEX concentrations in the dry season during midday and morning were consistent with increased photochemical activity during these hours. Moreover, higher ambient temperatures coupled with calm periods and low rainfall during the dry season could cause higher BTEX emissions. These studies also revealed that BTEX concentrations during dry season would be expected to be reduced by photochemical degradation, as this season is characterized by high solar radiation intensity 45 . Finally, the low wind speeds during dry season provided unfavorable dispersion conditions resulting in higher BTEX concentrations 45 . Jiang et al. 46 in their studies ordered the seasonal variation of the mean BTEX concentrations as follows: winter > spring > autumn > summer, pointing that besides sources' strength, the seasonal and diurnal variations of atmospheric BTEX in urban areas were also strongly dependent on meteorological conditions and photochemical activity.
As most of the studies presented above discussed the BTEX concentration changes in the ambient air in the vicinity of industrial sources and traffic emission it cannot be compared directly to our studies. In our research BTEX concentrations in the breathing zones were investigated, where the only source of emission was the presence of the refinery productions processes. Studies of Tabari et al. 25 in Mahshahr oil shipping port, Khuzestan province, Iran revealed that the total emission of VOCs were equal to 933.25 tons/year with the main emission sources to be storage tanks, pump houses, and wastewater treatment pool. BTEX emissions were equal to 1.49 tons/year of benzene, 3.2 tons/year of toluene, 0.57 tons/year of ethylbenzene, and 1.53 tons/year of xylenes 25 . Thus, seasonal variability of the BTEX concentrations in our studies were primarily related with the emissions in the refinery and local atmospheric conditions in the petrochemical processing.

BTEX concentrations and threshold limit values.
Mean exposure values of BTEX were compared with the threshold limit values (TLVs) in various workstations in summer and winter seasons (Fig. 2). The American Conference of Governmental Industrial Hygienists (ACGIH) recommends the following threshold limit values (TLVs) for exposure to BTEX compounds: benzene 60 mg/m 3 (0.5 ppm), toluene 175.37 mg/m 3 (20 ppm), ethylbenzene 86.84 mg/m 3 (20 ppm), and xylene 434.19 mg/m 3 (100 ppm) 47 . As the threshold limit www.nature.com/scientificreports/ values (TLVs) are recommended for 8 h of work per day and for 5 days of work per week, the Scala brief model was used for correcting the amount of threshold limit value -time-weighted average (TLV-TWA) if work duration was longer than 40 h per week 47 . The results of our investigations revealed that the mean values of exposure to benzene for repairmen and site men workstations were higher than recommended TLV equal to 1.60 mg/ m 3 for both summer and winter seasons. Considering all workers, the mean value of exposure to benzene was higher than TLV equal to 1.60 mg/m 3 only for the summer season. Our results stay in line with the findings of Moradpour et al. 8 , who investigated 40 VOCs in the Mahshar Petroleum industrial zone, Khuzestan province, Iran located about 100 km east from the investigated in our studies. In these studies, it was revealed that benzene concentrations in Mahshar Petroleum facility exceeded the threshold limit value for occupational exposure 8    www.nature.com/scientificreports/ Jalilian et al. 48 , who demonstrated in the study on BTEX effect on blood and spirometry parameters among staff in the Oil Refining Company in Iran, that toluene, ethylbenzene, and xylenes, concentrations in the breathing zones were lower than the TLV-TWA recommended by ACGIH, while the average concentration of benzene exceeded the allowable limit. Thus, it is recommended that facilities like petrochemical plants were equipped in extensive air monitoring to ensure that exposure is below occupational limits, especially for workplaces with higher levels of benzene, where the frequency of controls should be increased 49 . Various means of protective measures against exposure to BTEX compounds, like adequate engineering, management controls, and periodic inspection, are suggested to protect the employee's health 48 .
Non-carcinogenic risk assessment. The probability distributions and percentiles related to non-carcinogenic risk (HQ) of exposure to BTEX for various workstations in summer (Fig. 3) and in winter (Fig. 4) seasons were calculated using Monte Carlo simulation. In Table 3 (3.38) in winter season. For safetymen, the highest mean value belonged to benzene exposure in summer season (11.90) and in winter season (5.33). For repairmen also, the highest mean value was related to exposure to benzene in summer season (162.00) and winter season (139.00). For site  www.nature.com/scientificreports/ men, the highest mean HQ value was assigned to xylene exposure in summer season (227.00) and benzene in winter season (67.10). For all workers, the highest mean HQ values were related to xylene exposure in summer season (699.00) and in winter season (172.00). In all workstations, the mean HQ value in summer season was higher than in winter season. In summer season, the highest mean values of estimated HQ for benzene (162.00), toluene (2.23), and ethylbenzene (36.00) were related to exposure of repairmen, while the highest mean value of estimated HQ for xylene (227.00) belonged to exposure of site men. In winter season also, the highest mean values of estimated HQ were stated for benzene (139.00), toluene (1.81), and ethylbenzene (12.4), while the highest mean value of estimated HQ for xylene (51.60) belonged to exposure of site men.
Considering HQ values in occupational exposure to BTEX in refinery industry it was revealed that all calculated values indicated moderate to serious non-carcinogenic risk. Only for supervisors and safetymen exposure to toluene, for all workers in winter season, and for ethylbenzene exposure for safety men in winter season the risk was negligible.
Carcinogenic risk assessment. The probability distributions and percentiles related to carcinogenic risk (LCR) of exposure to BTEX for various workstations in summer (Fig. 5) and in winter (Fig. 6) seasons were also performed using Monte Carlo simulation. In Table 4 the statistical description of LCR values for BTEX in various workstations in summer and in winter seasons as a heat map was presented. The results showed that the mean LCR values for benzene and ethylbenzene were higher than 1 × 10 -4 for all workstations in both summer and winter seasons. It indicated that there was a definite carcinogenic risk for workers exposed to benzene and ethylbenzene in all workstations of the investigated company in both seasons. Due to the lack of SF values for toluene and xylene from toxicological databases, only calculations for benzene and ethylbenzene were presented.
Results indicated that the highest mean LCR value for supervisors was related to ethylbenzene exposure in summer (6.85 × 10 -3 ) and in winter (3.95 × 10 -3 ) seasons. For safetymen, the highest mean LCR value belonged to ethylbenzene exposure in summer season (8.48 × 10 -3 ) and to benzene exposure (3.70 × 10 -3 ) in winter season. For repairmen also, the highest mean value was related to benzene exposure in summer season (5.42 × 10 -2 ) and ethylbenzene in winter season (6.80 × 10 -2 ). For site men, the highest mean value was assigned to benzene exposure in summer (3.25 × 10 -2 ) and in winter (1.73 × 10 -2 ) season. For all workers, the highest mean LCR values were related to ethylbenzene in summer (3.55 × 10 -2 ) and in winter (3.16 × 10 -2 ) season. Considering all workstations, the mean LCR value in summer season was higher than in winter season.
In summer season, the highest mean values of estimated LCR for benzene (5.42 × 10 -2 ) and ethylbenzene (1.14 × 10 -1 ) were related to workstation of repairmen. In winter season also, the highest mean value of estimated LCR for benzene (1.73 × 10 -2 ) was related to site men, while the highest mean value of estimated LCR for ethylbenzene (6.80 × 10 -2 ) belonged to repairmen.
Considering all LCR values in occupational exposure to BTEX in refinery industry it was revealed that all calculated values indicated definite carcinogenic risk, namely for all workstation, for benzene and ethylbenzene compounds, and for both summer and winter seasons.   www.nature.com/scientificreports/ BTEX in petrochemical plant in Serbia revealed that on average the employees were at high carcinogenic risk with a potential risk value equal to 6.10 × 10 -3 , with special concern to workers exposed to benzene concentrations higher than 18 mg/m 3 .
Research of Tong et al. 23 on health risk related with exposure to VOCs in a petrochemical refinery in Hainan, China revealed that benzene, toluene, ethyl benzene, and xylene were the primary pollutants. For non-carcinogenic risk the highest HQ values were related with the aromatic hydrocarbon extraction device (AHED).    3 in major oil distribution company in Iran revealed that the mean carcinogenic risk related with exposure to benzene were 16.08 for tanker loading workers, 2.47 for tank-gauging workers, 0.20 for drivers, 0.21 for firefighters, and 0.06 for office workers. Non-carcinogenic risk was revealed to be related to benzene and xylene exposure 3 . Results of Zhang et al. 2 performed in the petroleum refinery in the Pearl River Delta, China indicated the total non-carcinogenic risk in the basic chemical area the highest (3.1 × 10 3 ), owing to the highest level of total concentration of VOCs. The carcinogenic risk was reveled definite as the total carcinogenic risk ranged from 2.93 × 10 -3 in the wastewater treatment area to 1.1 × 10 -2 in the basic chemical area 2 . Investigations of Kitwattanavong et al. 26 analyzing the occupational exposure of petrol stations workers in the Inner City of Bangkok, Thailand revealed the high carcinogenic risk due to inhalational exposure as the risk levels for benzene were in the range 1.82 × 10 -4 -2.50 × 10 -4 , and for ethylbenzene: 4.11 × 10 -6 -5.52 × 10 -6 , exceeding the acceptable risk level.
On the contrary, some research did not confirm our findings. These were mainly related with the health risk analysis in the vicinity of the BTEX emission facilities, indicating that their concentrations were dispersed and thus not posed the significant health risk for inhabitants of these regions. Ercan et al. 44 indicated that the health risk assessment in the Istanbul city for benzene was equal to 7.71 × 10 -7 and non-carcinogenic risk for toluene, ethylbenzene, and xylenes was lower than the acceptable level of 1. In addition, health risk analysis performed among workers of the municipal wastewater treatment plant in Shiraz, Iran 7 indicated that carcinogenic risk values for all sampling sites were below the threshold limit of 1.0 × 10 -4 . Regarding the non-carcinogenic risk only for benzene the risk was exceeding the acceptable level of 1, while for toluene the risk values were lower than 1. The results of Zhang et al. 22 performed in Lanzhou, the major industrial areas of China revealed that noncarcinogenic risk related with airborne benzene exposure had acceptable HQ values for adults (0.185) and for children (0.102). However, a high carcinogenic risk (> 10 −4 ) from benzene in several sampling sites was revealed that diffuse distance become significant for carcinogenic risk.

Conclusions
Investigation on seasonal variations of BTEX concentrations in an Oil Refinery in Iran indicated that for all workstations investigated VOCs concentrations were higher in the summer than in the winter season. The decreasing order of BTEX concentrations in workstations was as follows: toluene > ethylbenzene > xylene > benzene. Benzene concentrations in breathing zones exceeded the threshold limit value TLV for repairmen and site men workstations for both summer and winter seasons. For all workers, the mean value of exposure to benzene was higher than TLV only in the summer season. Occupational inhalational exposure to all BTEX compounds revealed moderate to serious non-carcinogenic risk for all workstations in summer season. Carcinogenic risk from exposure to benzene and ethylbenzene in all workstations was stated as definite in both summer and winter seasons. Based on the results of our studies it is recommended to apply effective protection measures among workers of the oil refinery industry and to interest the responsible authorities and managers of this industry to introduce systemic solutions to protect employees' health.

Data availability
The datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request.